Re: sched: horrible way to detect whether a task has been preempted

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Apr 07, 2016 at 11:37:19PM +0200, Jiri Kosina wrote:
> On Thu, 7 Apr 2016, Jessica Yu wrote:
> 
> > Been sort of rattling my head over the scheduler code :-) Just following 
> > the calls in and out of __schedule() it doesn't look like there is a 
> > current flag/mechanism to tell whether or not a task has been 
> > preempted..
> 
> Performing the complete stack unwind just to determine whether task has 
> been preempted non-volutarily is a slight overkill indeed :/
> 
> > Is there any reason why you didn't just create a new task flag, 
> > something like TIF_PREEMPTED_IRQ, which would be set once 
> > preempt_schedule_irq() is entered and unset after __schedule() returns 
> > (for that task)? This would roughly correspond to setting the task flag 
> > when the frame for preempt_schedule_irq() is pushed and unsetting it 
> > just before the frame preempt_schedule_irq() is popped for that task. 
> > This seems simpler than walking through all the frames just to see if 
> > in_preempt_schedule_irq() had been called. Would that work?
> 
> Alternatively, without eating up a TIF_ space, it'd be possible to push a 
> magic contents on top of the stack in preempt_schedule_irq() (and pop it 
> once we are returning from there), and if such magic value is detected, we 
> just don't bother and claim unreliability.
> 
> That has advantages of both aproaches combined, i.e. it's relatively 
> low-cost in terms of performance penalty, and it's reliable (in a sense 
> that you don't have false positives).
> 
> The small disadvantage is that you can (very rarely, depending on the 
> chosen magic) have false negatives. That probably doesn't hurt too much, 
> given the high inprobability and non-lethal consequences.
> 
> How does that sound?

To do that from C code, I guess we'd still need some arch-specific code
in an asm() statement to do the actual push?

I think I'd prefer just updating some field in the task_struct.  That
way it would be simple and arch-independent.  And the stack walker
wouldn't have to scan for some special value on the stack.

-- 
Josh